allow label to be alias for text in button widgets

[tlambert03]

closes #109 by allowing label to serve as an alias for text on button widgets.

[codecov]

Codecov Report

Merging #150 (9c2d162) into master (aaa4eb9) will increase coverage by 0.01%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #150      +/-   ##
==========================================
+ Coverage   91.25%   91.26%   +0.01%     
==========================================
  Files          25       25              
  Lines        2503     2507       +4     
==========================================
+ Hits         2284     2288       +4     
  Misses        219      219              

Impacted Files	Coverage Δ
magicgui/widgets/_bases/button_widget.py	100.00% <100.00%> (ø)

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